This invention relates to a composite oxide having a novel crystal structure and exhibiting particularly excellent oxygen absorbing and desorbing capability which is utilizable for functional ceramics and for a catalyst for purifying exhaust gases.
Cerium oxide has hitherto been employed in large quantities as ceramics or as catalysts for purifying exhaust gases. In the field of catalysts, for example cerium oxide is used for enhancing the purification rate of exhaust gas components such as hydrocarbons, carbon monoxide and NO.sub.x, taking advantage of its characteristic properties of absorbing oxygen in an oxidative atmosphere and desorbing oxygen in a reducing atmosphere. In the field of ceramics, cerium oxide is used in the form of compounds or mixtures with other elements as electrically conductive ceramics, such as solid electrolytes, taking advantage of its characteristic properties mentioned above. Such cerium oxide heretofore known is usually prepared, for example by adding oxalic acid or ammonium bicarbonate to a solution of nitrate or chloride of cerium, filtering and washing the resultant precipitate and drying followed by calcining.
However, the composite oxide prepared by the above method predominantly composed of cerium oxide has a drawback that it is incapable of sufficiently absorbing and desorbing oxygen at 400.degree. to 700.degree. C., and is deteriorated in performance after heating to a higher temperature, even though it exhibits the oxygen absorbing and desorbing capability.
Up to now, cerium-zirconium composite oxide exhibiting high oxygen absorbing and desorbing capability is known and disclosed in Japanese Laid-Open Patent Application No.5-105428 (1993). Detailed scrutiny into the crystal structure of such oxide by Comparative Examples given hereinbelow has revealed that the structure is a mixed phase of a CaF.sub.2 -related structure phase, an h-tetragonal phase and a monoclinic phase (see FIG. 6). On the other hand, a tetragonal .PHI. phase has been known as a crystal phase (V. Longo and D. Minichelli, J. Amer. Ceramic Soc., 56(1973), 600.; P. Duran, M. Gonzalez, C. Moure, J. R. Jurado and C. Pascual, J. Materials Sci., 25(1990), 5001). It is reported that the .PHI. phase is formed after annealing at 993K for several months, and that equal amounts of h-tetragonal phase and CaF.sub.2 -related structure phase are contained in the formed .PHI. phase. It is also reported in the above literature that this .PHI. phase is decomposed at elevated temperatures. That is, researches are currently conducted for composite oxides exhibiting various oxygen absorbing and desorbing capability based upon difference in the crystal structure, and development of composite oxides exhibiting more excellent oxygen absorbing and desorbing capability has been desired.